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I'm using the volume direct pass for a smoke sim I have but that seems to give me the image of the smoke instead of a mask of where the smoke is and isn't so it becomes a problem when I'm trying to make a pass for purely black smoke. is there a way around this? Gray smoke and black smoke as examples

I know I could make the smoke white but that will effect the reflections on other objects in the scene

gray smoke

black smoke

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  • $\begingroup$ I don’t remember… does the smoke pass have an alpha channel that can be separated from it? It looks like it does from the preview. $\endgroup$
    – TheLabCat
    Commented Jun 29 at 22:02
  • $\begingroup$ @TheLabCat I'm not at a PC right now to check, but as far as I know you're right, this should be alpha transparency. It uses the max bounces for Transparent, which is for alpha channel transparency. But of course it needs to be rendered over a transparent background. desperrrr: VolumeDir only shows (like DiffDir for diffusion) the direct color, not any bounced light (that's the "...Ind" channels) or transparencies (that's the alpha channel). $\endgroup$
    – Gordon Brinkmann
    Commented Jun 29 at 22:31
  • $\begingroup$ I clicked the display alpha channel in the top right and the whole thing turned out white although i have figured out a makeshift solution which i will write an answer for once i have some time $\endgroup$
    – desperrrr
    Commented Jun 29 at 23:16
  • $\begingroup$ @desperrrr Sure everything turned out white. The Alpha output shows the tranparent areas of the image, not just the smoke. That's why I said you have to render it over a transparent background: smoke alpha. Then you can use that alpa to put something else in the background: replace alpha $\endgroup$
    – Gordon Brinkmann
    Commented Jun 30 at 0:48

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